1. Field of the Invention
The present invention relates to a method for manufacturing a power module substrate which is used for a semiconductor device controlling high electrical current and high voltage, a power module substrate, and a power module.
This application is based on and claims priority from Japanese Patent Application No. 2007-299385, filed on Nov. 19, 2007, the contents of which are incorporated herein by reference.
2. Background Art
In semiconductor elements, it is known that a power module used for supplying electrical power has an amount of heat generation, and the amount of heat generation is relatively high.
Consequently, as a substrate on which the power module is mounted, for example, a power module substrate is used in which a metal plate made of Al (aluminum) is joined to a ceramics substrate made of AlN (aluminum nitride) with an Al—Si (silicon) based brazing filler metal interposed therebetween.
In addition, the metal plate is formed as a circuit layer, a semiconductor chip of the power element is mounted on the metal plate with a solder material interposed therebetween.
Moreover, a power module has been proposed, in which a metal layer to which a metal plate made of Al or the like is joined is also formed on a lower face of the ceramics substrate for heat radiation and in which a whole power module substrate is joined to a heat radiation plate with the metal layer interposed therebetween.
Conventionally, a technique has been disclosed in, for example, Japanese Unexamined Patent Application, First Publication No. 1103-234045, in which the roughness surface of an AlN ceramics substrate is less than 0.5 μm in order to obtain an excellent joint strength between a metal plate which serves as the circuit layer and the metal layer and a ceramics substrate.
However, when the metal plate is joined to the ceramics substrate, even if the roughness surface of the ceramics substrate is simply reduced, a sufficiently-high joint strength is not obtained and there is a disadvantage in that the reliability thereof cannot be improved.
Even if, for example, a honing treatment is performed on the surface of the ceramics substrate by use of Al2O3 particles in a dry method and the roughness surface Ra thereof is made 0.2 μm, it was found that a peeling occurs at an interface thereof in a peeling test.
In addition, even if the roughness surface Ra is made less than or equal to 0.1 μm by use of a polishing method, there is a case where the peeling occurs at the interface in a similar manner.
Specifically, in recent days, in the power module, downsizing and reducing the thickness advances, and the usage environment becomes severe. Therefore, electronic components having a high amount of heat generation are generally used further.
Consequently, it is necessary to dispose a power module substrate on a heat radiation plate as described above.
In this case, since the power module substrate is rigidly fixed by the heat radiation plate, a large shear force is generated at a joint interface between the metal plate and the ceramics substrate when the substrate is subjected to thermal cycles.
As a result, improvement of the joint strength and reliability are further required.